This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The combustion of fuel within a combustor, e.g., integrated with a gas turbine, can be controlled by monitoring the temperature of the exhaust gas. At full load, typical gas turbines adjust the amount of fuel introduced to a number of combustors in order to reach a desired combustion gas or exhaust gas temperature. Conventional combustion turbines control the oxidant introduced to the combustors using inlet guide vanes. Under a partial load, the amount of oxidant introduced to the combustor is reduced and the amount of fuel introduced is again controlled to reach the desired exhaust gas temperature. Further, at a partial load, the efficiency of gas turbines drops because the ability to reduce the amount of oxidant is limited by the inlet guide vanes, which are only capable of slightly reducing the flow of oxidant. Further, the oxidant remains at a constant lower flow rate when the inlet guide vanes are in their flow restricting position. The efficiency of the gas turbine then drops when it is at lower power production because to make that amount of power with that mass flow a lower expander inlet temperature is required. Moreover, existing oxidant inlet control devices may not allow fine flow rate control and may introduce large pressure drops with any restriction on the oxidant flow. With either of these approaches to oxidant control, there are potential problems with lean blow out at partial load or reduced pressure operations.
Controlling the amount of oxidant introduced to the combustor can be desirable when an objective is to capture carbon dioxide (CO2) from the exhaust gas. Current carbon dioxide capture technology is expensive due to several reasons. One reason is the low pressure and low concentration of carbon dioxide in the exhaust gas. The carbon dioxide concentration, however, can be significantly increased from about 4% to greater than 10% by operating the combustion process under substantially stoichiometric conditions. Further, a portion of the exhaust gas may be recycled to the combustor as a diluent in order to control the temperature of the gas within the combustor and of the exhaust gas. Also, any unused oxygen in the exhaust gas may be a contaminate in the captured carbon dioxide, restricting the type of solvents that can be utilized for the capture of carbon dioxide.
In many systems, an oxidant flow rate may be reduced by altering the operation of a separate oxidant system. For example, an independent oxidant compressor may be throttled back to a slower operating speed thereby providing a decreased oxidant flow rate. However, the reduction in compressor operating speed generally decreases the efficiency of the compressor. Additionally, throttling the compressor may reduce the pressure of the oxidant entering the combustor. In contrast, if the oxidant is provided by the compressor section of the gas turbine, reducing the speed is not a variable that is controllable during power generation. Large gas turbines that are used to produce 60 cycle power are generally run at 3600 rpm. Similarly, to produce 50 cycle power the gas turbine is often run at 3000 rpm. In conventional gas turbine combustor operations the flow of oxidant into the combustor may not warrant significant control because the excess oxidant is used as coolant in the combustion chamber to control the combustion conditions and the temperature of the exhaust gas. A number of studies have been performed to determine techniques for controlling combustion processes in gas turbines with the intent of minimizing oxygen and undesirable combustion by-products, such as carbon monoxide, in the exhaust.
For example, International Patent Application Publication No. WO/2010/044958 by Mittricker et al. discloses methods and systems for controlling the products of combustion, for example, in a gas turbine system. One embodiment includes a combustion control system having an oxygenation stream substantially comprising oxygen and CO2, then mixing the oxygenation stream with a combustion fuel stream and combusting in a combustor to generate a combustion products stream having a temperature and a composition detected by a temperature sensor and an oxygen analyzer, respectively. The data from the sensors are used to control the flow and composition of the oxygenation and combustion fuel streams. The system may also include a gas turbine with an expander and having a load and a load controller in a feedback arrangement.
International Patent Application Publication No. WO/2009/120779 by Mittricker et al. discloses systems and methods for low emission power generation and hydrocarbon recovery. One system includes integrated pressure maintenance and miscible flood systems with low emission power generation. Another system provides for low emission power generation, carbon sequestration, enhanced oil recovery (EOR), or carbon dioxide sales using a hot gas expander and external combustor. Another system provides for low emission power generation using a gas power turbine to compress air in the inlet compressor and generate power using hot carbon dioxide laden gas in the expander.
U.S. Patent Application Publication No. 2012/0023954 by Wichman discloses a power plant and a method of operation. The power plant and method includes a main air compressor and a gas turbine assembly. The assembly includes a turbine combustor for mixing compressed ambient gas with a recirculated low oxygen content gas flow and a fuel stream to form a combustible mixture. The combustible mixture is burned in the turbine combustor, forming the recirculated low oxygen content flow. The assembly includes a recirculation loop for recirculating the recirculated low oxygen content gas flow from the turbine to the turbine compressor. The assembly also includes an integrated inlet bleed heat conduit that fluidly connects the gas turbine assembly to an input of the main air compressor and delivers a portion of the recirculating low oxygen content gas flow from the gas turbine assembly to the input of the main air compressor.
U.S. Pat. No. 8,205,455 to Popovic discloses a power plant and method of operation. The power plant includes a main air compressor and an oxidizer unit configured to deliver a compressed oxygen-rich gas flow to a gas turbine assembly. Each gas turbine assembly includes a turbine combustor for mixing the compressed oxygen-rich gas flow with a recirculated gas flow and a fuel stream to burn a combustible mixture and form the recirculated gas flow. The assembly also includes a recirculation loop for recirculating the recirculated gas flow from a turbine to a turbine compressor. The assembly further includes a recirculated gas flow extraction path for extracting a portion of the recirculated gas flow from the assembly and delivering this to a gas separation system. The gas separation system separates the portion of the recirculated gas flow into a nitrogen portion and a carbon dioxide portion.
U.S. Pat. No. 5,355,668 to Weil et al. discloses a catalyst-bearing component of a gas turbine engine. Catalytic materials are formed on components in the gas flow path of the engine, reducing emissions of carbon monoxide and unburned hydrocarbons. The catalytic materials are selected from the noble metals and transition metal oxides. The portions of the gas flow path where such materials are applied can include the combustor, the turbine, and the exhaust system. The catalytic coating can be applied in conjunction with a thermal barrier coating system interposed between a substrate component and the catalytic coating.
Past efforts to control the exhaust gas components have focused on reducing the content of standard pollutants, such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons. The gains have mostly been achieved by the use of near stoichiometric combustion with some catalysts, such as catalysts selected to reduce carbon monoxide and NOx in the exhaust. As a result, the concentration of various other gases in the exhaust, such as oxygen, may be higher than desirable.